Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 https://doi.org/10.1186/s12891-020-03333-y

STUDY PROTOCOL Open Access In vivo assessment of the elastic properties of women’s pelvic floor during pregnancy using shear wave elastography: design and protocol of the ELASTOPELV study Bertrand Gachon1,2,3*, Xavier Fritel1,3,4, Fabrice Pierre1 and Antoine Nordez2,5

Abstract Background: Animal studies have reported an increase in pelvic floor muscle stiffness during pregnancy, which might be a protective process against perineal trauma at delivery. Our main objective is to describe the changes in the elastic properties of the pelvic floor muscles (levator ani, external anal sphincter) during human pregnancy using shear wave elastography (SWE) technology. Secondary objectives are as follows: i) to look for specific changes of the pelvic floor muscles compared to peripheral muscles; ii) to determine whether an association between the elastic properties of the levator ani and perineal clinical and B-mode ultrasound measures exists; and iii) to provide explorative data about an association between pelvic floor muscle characteristics and the risk of perineal tears. Methods: Our prospective monocentric study will involve three visits (14–18, 24–28, and 34–38 weeks of pregnancy) and include nulliparous women older than 18 years, with a normal pregnancy and a body mass index (BMI) lower than 35 kg.m− 2. Each visit will consist of a clinical pelvic floor assessment (using the Pelvic Organ Prolapse Quantification system), an ultrasound perineal measure of the anteroposterior hiatal diameter and SWE assessment of the levator ani and the external anal sphincter muscles (at rest, during the Valsalva maneuver and during pelvic floor contraction), and SWE assessment of both the biceps brachii and the gastrocnemius medialis (at rest, extension and contraction). We will collect data about the mode of delivery and the occurrence of perineal tears. We will investigate changes in continuous variables collected using the Friedman test. We will look for an association between the elastic properties of the levator ani muscle and clinical / ultrasound measures using a Spearman test at each trimester. We will investigate the association between the elastic properties of the pelvic floor muscles and perineal tear occurrence using a multivariate analysis with logistic regression. Discussion: This study will provide original in vivo human data about the biomechanical changes of pregnant women’s pelvic floor. The results may lead to an individualized risk assessment of perineal trauma at childbirth. Trial registration: This study was registered on https://clinicaltrials.gov on July 26, 2018 (NCT03602196). Keywords: Perineal trauma, Shear wave elastography, Pregnancy, Levator ani muscle, Anal sphincter, Childbirth, Obstetric anal sphincter injury

* Correspondence: [email protected] 1Department of obstetrics and gynecology, Poitiers university hospital, 2 rue de la Miletrie CS90577, 86021 Poitiers Cedex, France 2Nantes Université, Movement - Interactions – Performance, MIP, EA 4334, F-44000 Nantes, France Full list of author information is available at the end of the article

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Background increase in elastic modulus may be a protective Perineal trauma is a frequent complication of child- process from perineal trauma. On rats, studies re- birth, which may lead to several pelvic floor disorders, ported an increase during pregnancy in both fiber such as anal incontinence, urinary incontinence, pelvic length and stiffness measured at a given sarcomere organ prolapse and sexual dysfunction [1–4]Inthe length [9, 10]. These changes could be interpreted most severe cases, perineal trauma could involve an consequently from the increase of loading. The in- obstetric anal sphincter injury (OASI) (rupture of the crease in fiber length was thought as a mechanism to external anal sphincter and, worse still, opening of limit the fiber strain that can cause injury. The in- the rectal mucosae) and/or levator avulsion. OASI, crease in stiffness was thought to be related to extra- which occurs in nearly 5% of first deliveries, is associ- cellular matrix content and would likely reduce the ated with postnatal anal incontinence and dyspareunia risk of injury due to large strain that occurs during [1]. Levator avulsion, which occurs in nearly 10% of parturition [5, 9]. This is supported by a higher ul- first deliveries, is associated with pelvic organ pro- timate stress in biological tissues that have higher lapse and sexual dysfunction [3]. These injuries are stiffness [12]. These data about animal experimenta- associated with trauma of the pelvic floor muscles at tion need to be read with caution because there is no vaginal delivery during which these muscles are over- data proven that these phenomena occur in a same stretched, up to three times their initial length [5]. way in women. Several risk factors are described in the literature To date, several techniques have been described to as- (forceps delivery, fetal macrosomia, etc.) However, the sess the in vivo elastic properties of the pelvic floor mus- occurrence of these complications remains very diffi- cles (vaginal elastometry, tactile imaging, elastography) cult to predict [1–4]. It is likely that the risk of pelvic [13, 14]. One of the most relevant techniques is shear floor trauma can be influenced by intrinsic character- wave elastography (SWE), which allows a direct, quanti- istics of the pelvic floor muscles and their ability to tative and noninvasive assessment of the muscles [15]. lengthen sufficiently to enable passage of the fetus Recently, we reported the feasibility of an in vivo assess- through the birth canal without being damaged. Iden- ment of the elastic properties of the levator ani muscle tifying women with a high risk of perineal trauma using this technique (100% of procedures allowing a antenatally would enable clinicians to propose individ- visualization of the levator ani muscle and a measure of − ualized counseling and preventive strategies for these elastic properties in women with a lower than 35Kg.m 2 women. body mass index) [13]. Few studies have indicated that some intrinsic bio- In accordance with animal experimentation, we mechanical characteristics of pregnant women could hypothesize that there are changes in elastic properties be associated with the risk of perineal trauma [6–8]. of women’s pelvic floor through pregnancy and that In a recent prospective study, we reported an associ- SWE is relevant to follow these changes [9, 10]. They ation between peripheral ligamentous laxity (assessed might be specific to pelvic floor muscles without, or less, at the metacarpophalangeal joint) and the risk of significant changes for peripheral muscles such as biceps OASI. In that study, the women with the greatest brachii or gastrocnemius medialis. Finally, the hypothet- ligamentous laxity had the greatest risk of OASI [8]. ical changes in elastic properties of women’s pelvic floor This result supports the hypothesis of an association may be associated with their intrinsic risk of perineal between a woman’s individual biomechanical charac- trauma at childbirth. Indeed, during vaginal delivery, a teristics and her risk of perineal trauma. However, the major strain is applied to pelvic floor muscles which are main limitation of this study was that it was designed stretched up to 3 times their initial length [5, 16]. Thus, to analyze data about an upper limb joint, which is intrinsic elastic properties of pelvic floor muscles may be probably very different from pelvic floor muscle tis- associated with their ability to support this strain with- sues [8]. out being damaged. Data about changes in intrinsic characteristics of Our main objective is to describe the changes in the women’s pelvic floor muscles during pregnancy has elastic properties of the pelvic floor muscles (levator ani, mainly originated from experiments on rats [9–11]. external anal sphincter) during human pregnancy using Some authors have reported that an increase in mus- shear wave elastography (SWE) technology. The second- cular fiber length and an increase in pelvic floor ary objectives are as follows: i) to look for specific muscle stiffness occurs during pregnancy, while no changes of the pelvic floor muscles compared to periph- changes were reported for peripheral muscles [9–11]. eral muscles; ii) to determine whether an association be- This could be explained by the increase in mechanical tween the elastic properties of the levator ani and loading (force due to gravity of growing fetus) applied perineal clinical and B-mode ultrasound measures exists; to pelvic floor muscles during pregnancy [10]. This and iii) to provide explorative data about an association Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 3 of 12

between pelvic floor muscle characteristics and the risk Population of perineal tears. The inclusion criteria are as follows: women older than 18 years, volunteers, nulliparous, with a normal singleton Methods pregnancy, and who benefit from health insurance. Design The exclusion criteria are as follows: women with pre- The ELASTOPELV study will be a prospective, longitu- vious vaginal and/or cesarean delivery, women with a dinal, monocentric study. The scheme of the study will personal history of pelvic floor disorders (urinary incon- involve 3 visits during pregnancy: the first one between tinence, anal incontinence, pelvic organ prolapse), 14 and 18 weeks, the second between 24 and 28 weeks women with a body mass index (BMI) higher than 35 − and the last between 34 and 38 weeks of pregnancy kg.m 2, women with chronic muscular disease, women (Fig. 1). For each of these three visits, the protocol will requiring admission into a psychiatric unit, women follow these steps: clinical perineal assessment, ultra- under judicial protection, and women unable to under- sound B-mode perineal assessment, SWE assessment of stand the French language. the levator ani muscle, the external anal sphincter, the If an included woman has a pregnancy who became biceps brachii muscle and the gastrocnemius medialis pathological (define by the necessity of follow-up into muscle (Fig. 1). pathological pregnancies consultations and/or admission in pathological pregnancies unit) she will no longer par- Setting ticipate to the study and no data will be collected after The study will take place in the department of Obstet- this event. If a woman wants to stop its participation, no rics and Gynecology of the Poitiers University Hospital, more data will be collected. Poitiers, France.

Fig. 1 ELASTOPELV study design, inclusion criteria and data collected Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 4 of 12

Power calculation In anisotropic solid the eq. E = 3 μ is no more valid. This study deals with exploratory data with an ab- So, measurements should be divided by a factor 3 to ob- sence of previous data that would allow a power cal- tain measurement of the muscle shear modulus [15, 22]. culation. Furthermore, the main endpoint of the A previous study has demonstrated that the shear modu- project is descriptive (to describe changes in the elas- lus is strongly and linearly related to the Young modu- tic properties of the levator ani muscle during preg- lus, which supports the relevance of shear modulus nancy), and, therefore, an a priori power calculation measurements obtained with the Aixplorer® device for does not appear necessary. We aim to obtain and the study of muscle biomechanics [15, 23]. study the data from at least 50 women. We consid- SWE is based on the hypothesis of a linear elasticity ered this sample size in part due to previous studies that is commonly assumed in both magnetic resonance that reported an increase in levator hiatus area and elastography and ultrasound SWE. A lot of SWE studies ligamentous laxity during pregnancy, as well as analyzed the effects of loading on changes in muscle changes in the intrinsic biomechanical characteristics elasticity [15]. The effect of nonlinear elasticity should of pregnant women, from between 20 to 50 women be studied in the future. [17–19]. We estimate that 20% of the women will be excluded during pregnancy because of a complicated Safety pregnancy and/or their own choice, leading to an ob- The protocol will be performed with a commercialized jective of 60 inclusions. ultrasound scanner. This is considered a noninvasive and very safe examination [24]. The technology is widely used to assess the elastic properties of peripheral mus- Recruiting procedure cles without any adverse outcomes [15, 25]. Previous Women eligible for the ELASTOPELV study will be in- studies have reported the use of SWE during pregnancy formed about the study during their clinical consulta- for both mother and fetal tissue assessment without any tions and/or ultrasound consultations during a normal adverse outcomes. Therefore, the use of SWE for the as- pregnancy follow-up by their obstetrician and/or mid- sessment of the pelvic floor muscles of pregnant women wife. Eligible women interested in this study will be con- is safe [26, 27]. tacted by the investigator to obtain more information about the study and proceed with the inclusion if they Data collection give their free informed consent. Women’s characteristics At the first visit, after validation of the inclusion and ex- Shear wave elastography principles clusion criteria, we will collect anthropometric data The novelty of the ELASTOPELV study is based on the about the women: height (in cm), weight (in kg) and − use of SWE to investigate the in vivo elastic properties BMI (in kg.m 2). Demographic data and obstetric his- of the pelvic floor muscles of pregnant women. SWE al- tory will also be collected during the first visit: age (in lows a quantitative in vivo assessment of tissues during a years), gestity, and verification of the absence of a previ- classic ultrasound examination [15, 20]. An Aixplorer® ous delivery (cesarean or vaginal). The dominant side device (Supersonic Imagine, Aix-en-Provence, France) will be recorded: right-handed or left-handed. will be used. A remote mechanical perturbation is ap- plied to the tissue using a specific ultrasound sequence Clinical pelvic floor assessment to induce the propagation of a shear wave into the tissue We will perform a clinical pelvic floor assessment at of interest. Due to the ultrafast ultrasound acquisition, each visit. This examination will follow the recommen- the wave’s propagation speed is measured perpendicular dation of the Pelvic Organ Prolapse Quantification sys- to the ultrasound beam. This shear wave speed propaga- tem (POP-Q) [28]. We will perform the procedure with tion is linked with the elastic modulus of the tissue: the women in the lithotomy position after voiding and max- stiffer the tissue, the higher the wave’s propagation speed imal strain on the Valsalva maneuver. The position of is [15, 20, 21]. The elastic properties of the tissue are re- each point of the POP-Q will be expressed in negative ported as the Young modulus, which represents the link or positive values (in cm), and the length of each seg- between a stress and a strain in an isotropic tissue (simi- ment of the POP-Q (genital hiatus (gh), perineal body lar mechanical properties in all directions). Muscles are (pb), total vaginal length (tvl)) will also be expressed in stiffer along the fiber direction and thus cannot be con- centimeters [28]. sidered isotropic. Considering an isotropic solid, the Aixplorer device gives E (Young’s modulus) as a meas- Ultrasound B-mode pelvic floor assessment urement with, E = 3 μ = ρV2. with μ the shear modulus, ρ We will perform an ultrasound B-mode pelvic floor as- the density, V the shear wave speed. sessment at each visit of the study. This examination is Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 5 of 12

performed with the woman in the lithotomy position One single investigator will perform all the after voiding. We will use an Aixplorer® device with an measurements. XC6–11–6 MHz abdominal curved probe (V12, Super- sonic Imagine, France). We will measure the anteropos- – Specificity for the levator ani muscle assessment terior hiatal diameter (distance between the antero- inferior extremity of the pubic symphysis and the ano- For this assessment, we will use the procedure that we rectal junction, in cm) at rest, during a maximal strain described for nonpregnant women in a previous publica- on the Valsalva maneuver and at maximal perineal con- tion [13]. The examination will be performed with the traction. For these measures, we will use the translabial woman in the lithotomy position after voiding. We will perineal ultrasound approach widely described by Dietz first locate the levator ani muscle at its pubic insertion et al. [29, 30]. We will ask women to perform two initial during a B-mode ultrasound using the procedure de- Valsalva maneuvers with biofeedback instruction to pre- scribed by Dietz et al. for the diagnosis of levator avul- vent levator coactivation from serving as a confounding sion, with 87% agreement between observers. We will factor in our analysis [31]. place the probe in the sagittal plane on the perineum and apply a 10° parasagittal inclination to identify the Shear wave elastography assessments muscle (Fig. 2)[32]. We will perform assessments during As previously stated, an assessment of the elastic the three considered conditions: rest, stretch and sub- properties of the pelvic floor muscles of pregnant jective maximal contraction. For the stretch condition, women will be performed at each visit using SWE. the woman will be asked to perform a maximal Valsalva These measures will be performed for the levator ani, maneuver. We will prevent levator coactivation in the the external anal sphincter, biceps brachii and gastro- same way that we described for ultrasound pelvic floor cnemius medialis muscles. Each measurement will be assessment [31]. With this procedure, a previous study performed on the right side of the woman, as it reported that the shear modulus measured in levator ani would be ideal to obtain all the measurements for the muscle in non pregnant women is about 16 kPa at rest same side, preferentially while the women are in left and 35 kPa during Valsalva maneuver [13]. lateral decubitus, which offers the possibility of acces- sing the right limbs. For each muscle’s location, we – Specificity for external anal sphincter assessment will investigate the muscle during three conditions: rest, stretch and subjective maximal contraction. We The woman’s position will be the same as for the le- will use an Aixplorer® device (V12, Supersonic Im- vator ani muscle. We will place the probe on the peri- agine, France) with a linear SL 18–5 probe (5–18 neum immediately above the anus in the axial plane MHz). Every measure will begin by performing a B- (Fig. 3). We will first locate the external muscle using a mode procedure to locate the muscle. Then, we will B-mode ultrasound and then proceed to the SWE as- proceed with recording a 10-s video clip of the SWE sessments in the middle of the anterior zenith of the measurements. The region of interest will be outlined sphincter ring for the three conditions: rest, maximal by hand and the measure of the shear modulus will Valsalva maneuver and subjective maximal perineal con- be obtained within this region in postprocessing. For traction [33]. assessment at rest and during a stretch, we will con- sider the mean shear modulus of the video clip, – Specificity for biceps brachii muscle assessment whereas for assessment during contraction, we will consider the maximal shear modulus. First, we will identify the proximal and distal insertion We will perform three measures for each condition of the biceps brachii using B-mode ultrasound and per- (rest, stretch and contraction) and consider the mean form SWE acquisition midway between these insertions of the three measures for analysis. We choose to con- for three conditions: rest, standardized extension, and sider the mean of the 3 acquisitions for each condi- subjective maximal contraction. We will perform an as- tion to maximize the reliability of the measurement sessment at rest performed with the upper limb having a by considering all the measures (the most intense and 90° flexion of the elbow, which will be at the same height the weakest contraction, the first measure after instal- as the shoulder, with the hand in the pronation position. lation, etc.) The forearm will rest on a flat support, allowing the bi- As previously mentioned, we will measure the Young ceps brachii to be totally free and accessible (Fig. 4A). modulus using the Aixplorer® device, which will be di- We will systematically verify the 90° flexion of the elbow vided by a factor of 3 to obtain the shear modulus, using a digital goniometer. For the assessment during which is more accurate for anisotropic tissues such as extension, the position will be the same but with a 180° muscle [23]. extension of the elbow (verified with the digital Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 6 of 12

Fig. 2 Levator ani muscle SWE assessment: probe position and example of acquisition goniometer) and the hand in the pronation position (Fig. distal insertions of the muscle for the three conditions: 4b). Finally, for the measurements during contraction, rest, standardized extension, and subjective maximal we will ask the woman to have a subjective maximal contraction. For the assessment at rest, the left leg will contraction of her biceps brachii in the rest assessment be flexed, the right leg will be fully extended (180°, veri- position. With this procedure, a previous study reported fied with the digital goniometer) and the ankle will be in that the shear modulus measured in biceps brachii a neutral position (Fig. 5a). For the measurement during muscle in non pregnant volunteer is about 3 kPa at rest extension, the woman will be in the same position but and 19 kPa when stretched [25, 34]. with the right foot supported on a 20° inclined plane to apply a standardized extension of the gastrocnemius – Specificity for gastrocnemius medialis muscle medialis (Fig. 5b). Finally, we will proceed to obtain the assessment measurement during contraction with the woman in the same position as for the assessment at rest but with a Usually, this measure is performed with the volunteer voluntary maximal contraction of the gastrocnemius lying down in ventral decubitus. Because of the evident medialis. With this procedure, a previous study reported risks of compression of the gravid uterus, such a position that the shear modulus measured in gastrocnemius med- is not ideal for pregnant women, and so the assessments ialis muscle in non pregnant volunteer is about 3.1 kPa will be performed while the woman is in left lateral de- at rest [25]. cubitus. First, we will identify the proximal and distal in- sertions as well as the lateral borders of the – Data related to mode of delivery gastrocnemius medialis in B-mode ultrasound. We will perform the SWE acquisition midway between the lat- After childbirth, we will obtain the following data from eral borders and midway between the proximal and the subjects’ medical files: Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 7 of 12

Fig. 3 External anal sphincter SWE assessment: probe position and example of acquisition

– spontaneous or induced labor Analysis – epidural analgesia Judgment criteria – duration of the second stage of labor (time The primary judgment criteria will be the evolution of between full cervical dilatation and the birth, in the shear modulus of the pelvic floor muscle (levator ani minutes) and external anal sphincter) across the pregnancy – duration of the expulsive phase (time between the assessed at rest, during the Valsalva, and during a onset of pushing and the birth, in minutes) contraction. – mode of delivery (spontaneous vaginal delivery, Secondary judgment criteria will be: operative vaginal delivery, cesarean delivery) – indication for potential cesarean delivery (fetal – the association between POP-Q measurements (es- distress, other) pecially gh and pb) and the elastic properties (shear – type of instrument used for potential operative modulus) of the levator ani muscle at each visit; delivery (vacuum, forceps, spatulas) – the association between perineal B-mode ultrasound – indication for potential operative delivery (fetal measurements and the elastic properties (shear distress, other) modulus) of the levator ani muscle at each visit; – episiotomy use – the changes in shear modulus of the biceps – potential perineal tears classified according to the brachii and gastrocnemius medialis muscles Royal College of Obstetricians and Gynaecologists during pregnancy compared to the pelvic floor (RCOG) guidelines [35, 36]. muscles; Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 8 of 12

Fig. 4 SWE acquisitions of the biceps brachii muscle at rest and standardized extension

Fig. 5 SWE acquisitions of the gastrocnemius medialis muscle at rest and standardized extension Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 9 of 12

– the association among the shear modulus of the Ethical and reglementary considerations pelvic floor muscles (levator ani and external anal Every volunteer will receive oral and written information sphincter) at the last visit, the mode of delivery about the study and must give her free and informed (spontaneous vaginal delivery, operative vaginal written consent before any investigation. The study was delivery, cesarean delivery) and the potential approved by an ethical committee (Comité de Protection occurrence of a perineal tear (RCOG-WHO des Personnes Ile de France VIII) and is referenced with classification, French guidelines) [35, 36]. the ID RCB: 2018-A01422–53. The study is also regis- tered on https://clinicaltrials.gov (NCT03602196). Plan of analysis Availability of data and materials We will describe the anthropometric and sociodemo- Supporting data could be accessed on request to Poitiers graphic characteristics of the included women. Age, BMI University Hospital, Department of gynecology and Ob- and term of pregnancy will be reported as the mean and stetrics, France. standard deviation (SD). For all the other analysis, we will only consider data about women who completed the Discussion three planned visits. For each trimester, we will only col- Short summary of the study lect continuous variables, which will be reported as the It is difficult to predict the outcome of severe perineal means and SDs. Changes in these variables during preg- trauma (OASI and/or levator avulsion) at childbirth, as nancy will be investigated using a Friedman test. Obstet- there is a strong potential of an alteration of the ric data will be reported as the means and SD for woman’s health. One hypothesis to optimize the effi- continuous variables and as percentages and frequencies ciency of risk prediction is to consider the intrinsic bio- for categorical variables. mechanical characteristics of women’s pelvic floors. Regarding the study endpoints, we will first report the Such an approach may allow an individualized risk as- main outcome of this study, which consists of the sessment personalized information for each woman. Our changes in the pelvic floor muscle’s shear modulus prospective, monocentric, longitudinal study will include across the pregnancy. Then, we will report the changes 60 nulliparous pregnant women. Three visits are in all other measured pelvic floor-related parameters planned in this protocol (one per trimester of preg- (POP-Q measurements, ultrasound B-mode measure- nancy) and will include clinical (POP-Q) and ultrasound ments). We will look for an association between the assessment of the pelvic floor, SWE assessment of the shear modulus of the levator ani muscle and clinical pelvic floor muscles (levator ani, external anal sphincter) (POP-Q measurements) and ultrasound B-mode assess- and the biceps brachii and gastrocnemius medialis mus- ments at each trimester using a Spearman correlation cles. Finally, data about the mode of delivery (cesarean coefficient calculation. Second, we will report the section or vaginal delivery) and the occurrence of peri- changes in the shear modulus of the biceps brachii and neal tears will be collected. The main endpoint will be to gastrocnemius medialis muscles to look for changes be- describe the changes in the elastic properties of the pel- tween the different investigated locations. Third, we will vic floor muscles across pregnancy. The secondary end- look for an association between the shear modulus of points will be to look for an association between SWE the pelvic floor muscles (levator ani and external anal measurements of the levator ani muscle and clinical and sphincter) and both the mode of delivery (vaginal or ultrasound perineal assessments, to compare muscular cesarean delivery) and perineal tear occurrence using changes during pregnancy among the pelvic floor mus- univariate analysis. Variables with a level of significance cles and the biceps brachii and gastrocnemius medialis greater than p < 0.15 in univariate analysis will be in- muscles and to look for an association among the elastic cluded in the multivariate analysis using a logistic re- properties of the pelvic floor muscles, the mode of deliv- gression. We will perform statistical analysis with Stata ery and the occurrence of perineal tears. software (version V14IC; Stata Corporation, College Sta- tion, TX, USA). For all analyses, significance will be con- Justification of methodological choices sidered for p < 0.05, and we will calculate odds ratios Choice of shear wave elastography technology to (ORs) with 95% confidence intervals when appropriate. investigate pelvic floor muscles Few other techniques have been proposed for investi- Study duration gating the elastic properties of pelvic floor muscles. We have planned for an 18-month period of inclusion, Kruger et al. reported the use of a vaginal elastometer which led to a total study duration (from the inclusion to investigate the elastic properties of the levator ani of the first women to the end of the follow-up of the last muscle in both pregnant and nonpregnant women women) of 24 months. [14, 37]. This device consists of a vaginal speculum Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 10 of 12

with several force sensors, allowing the acquisition of Choice of investigating biceps brachii and gastrocnemius a force/displacement curve. Such a device is quite in- medialis muscles teresting, but because it measures the properties of We expect to study muscles with different characteris- both the vaginal wall and the levator ani muscle, the tics. Considering that the biceps brachii is not exposed measurements of the elastic properties of the levator to any increases in mechanical loading related to preg- ani muscle might be biased. Furthermore, we think nancy, we expect to find a different pattern compared to that the vaginal intrusion could be associated with a pelvic floor muscles. The difference might be less pro- lower participation rate since it involves the intromis- nounced for the gastrocnemius medialis since this sion of a medical device by an investigator. Egorov muscle is exposed to an increase of loading due to the et al. developed a vaginal tactile imaging device con- increase in weight that occurs during pregnancy. We sisting of a vaginal ultrasound probe supplemented also chose these two peripheral muscles because they with force and temperature sensors [38, 39].Sucha are superficial, large and easily accessible muscles. Fur- device is expected to provide an assessment of the thermore, we have data reporting that SWE is reliable to pelvic floor elastic properties. We consider that this investigate these muscles with high performance reliabil- technique presents the same limitations as the vaginal ity indicators [25]. elastometer of Kruger et al. [14, 37]. Static elastogra- phy is another ultrasound-related functional imaging Justification of inclusion and exclusion criteria technology that can be used to assess women’spelvic We choose to include only nulliparous women in this floors with a non-invasive approach [40–43]. How- study. This choice is easily understandable by the will- ever, this technique has major limitations in providing ingness to avoid bias related to any previous obstetrical non direct and qualitative assessments of the pelvic perineal trauma. The elastic properties of the pelvic floor floor. muscles that we will report in this study will be solely The choice of the transperineal approach to assess pel- related to the intrinsic characteristics of the woman and vic floor muscles is supported by an important literature the changes induced by the pregnancy. We will also ex- reporting that such an approach is efficient (in terms of − clude women with a BMI above 35 kg.m 2. This is due acceptability and reliability) to investigate pelvic floor to the results of a feasibility study that reported difficul- muscles [29, 30, 32, 33]. In 2018, our research team pub- ties in SWE assessments of the levator ani muscle dur- lished a feasibility study on the use of SWE to investigate ing the Valsalva maneuver for women with high BMIs the elastic properties of the levator ani muscle in non- [13]. These difficulties were due to a loss of visibility of pregnant women with this transperineal approach [13]. the levator ani during the maneuver using a superficial In this paper, we report that we were able to linear probe; the muscle became too deep to be clearly individualize the levator ani muscle and to measure a located. shear modulus in 100% of women with a lower than − 35Kg.m 2 BMI which allows to report the feasibility of the procedure. We consider that the fact that we investi- Expected results gate only the right levator ani muscle do not induce any Concordance with animal experimentations bias considering that in this previous study, we reported As we mentioned in the background section, human that there are no differences between the elastic proper- data about the evolution of pelvic floor muscles during ties of the right and left levator ani muscles, assessed pregnancy are lacking. It has been reported in animal ex- using SWE [13]. There is no published technique for periments that during pregnancy, the stiffness of the pel- investigating in vivo the elastic properties of the ex- vic floor muscle increases due to a drastic increase in ternal anal sphincter. Considering the easy access to total collagen content [9–11]. As we mentioned it in the the external anal sphincter using ultrasound with a background section, these animal experimentation re- transperineal approach and the efficiency of SWE in lated data must be interpreted carefully considering that other muscle applications, we consider that this there is no work with a confirmation that these phenom- choice is relevant [33]. In the future, this examination ena occur in a same way in pregnant women. could be easily performed in the ultrasound follow-up We expect to report a similar increase in the stiffness of pregnant women. We do not have data about the of the pelvic floor muscles during pregnancy in our reproducibility of pelvic floor muscles assessment study, which will support the data from animal experi- using SWE. Nevertheless, considering the easy access ments. Such an increase in stiffness might be a protect- to these muscles with a transperineal approach and ive process from perineal trauma given that tissue with the fact that SWE is a reliable tool for assessing per- the lowest stiffness easily raises their plasticity threshold ipheral muscles we expected a good reproducibility of to a level beyond which irreversible damage occurs in the technique. the tissue [12]. Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 11 of 12

One potential confounding factor for the interpret- in a biomechanical study, reduce the stress on the ation is that we do not have data about the use of peri- muscles and the force required to delivery success- neal stretching device such as Epi-No® during pregnancy. fully [47]. Our data might be helpful in providing a There is no data about the impact of such a practice on justification for implementing this type of study and pelvic floor muscle elastic properties. Some works re- offering the possibility of performing a power ports an increase in perineal extensibility, but it reports calculation. maximal vaginal compliance to the Epi-No® without data Abbreviations about a direct assessment of pelvic floor muscles stiff- BMI: Body Mass Index; Gh: Genital hiatus; OASI: Obstetric Anal Sphincter ness [44, 45]. Furthermore, its use is not recommended Injury; Pb: Perineal body; POP-Q: Pelvic Organ Prolapse Quantification; in French guidelines [36]. This considered, we think that RCOG: Royal College of Obstetricians and Gynaecologists; SD: Standard Deviation; SWE: Shear Wave Elastography; Tvl: Total vaginal length; the risk of bias in our cohort is weak. WHO: World Health Organization

Concordance between SWE measurements and clinical / Acknowledgments None. ultrasound measurements We expect to report an association between the elas- Authors’ contributions tic properties of the levator ani muscle and the clin- All authors have read and approved the manuscript. BG - Main text writing, main investigator, study design, methods elaboration. XF – Review of each ical and ultrasound assessments of pelvic floor version of the manuscript, methods elaboration (clinical aspects). FP - Review distension. For the clinical assessment, we will investi- of each version of the manuscript, methods elaboration (clinical aspects). AN gate all POP-Q measurements but with a special – Review of each version of the manuscript, methods elaboration (biomechanical aspects). interest in the gh and pb measurements that are per- formed during the Valsalva maneuver and that reflect Funding pelvic floor distension that occurs during the maneu- There is no funding for this study. ver. A similar association will be investigated for the Availability of data and materials ultrasound assessment (distance between the pubic Not applicable. This is a study protocol. symphysis and the anorectal angle). If we can report Ethics approval and consent to participate such a correlation between the elastic properties of Every volunteer receives an oral and written information about the study the levator ani investigated using SWE and clinical / and give her free and informed written consent before any investigations. ultrasound pelvic floor distension, it will support the The study is approved by an ethical committee (Comité de Protection des Personnes - Ile de France VIII: Hopital Ambroise Paré – 9 avenue Charles de efficiency and the applicability of SWE for functional Gaulle 92100 Boulogne-Billancourt, France) and referenced with the ID RCB: pelvic floor muscle assessments. We choose to focus 2018-A01422–53. The study is also registered on https://clinicaltrials.gov on the levator ani muscle for this analysis given the (NCT03602196). well-reported association between levator hiatus and Consent for publication pelvic organ mobility [46]. Not applicable.

Competing interests Preliminary data about the hypothetical association The authors declare that they have no competing interests. between elastic properties of women’s pelvic floors and obstetric perineal trauma Author details 1Department of obstetrics and gynecology, Poitiers university hospital, 2 rue Finally, we will look for a potential association be- de la Miletrie CS90577, 86021 Poitiers Cedex, France. 2Nantes Université, tween the elastic properties of the pelvic floor mus- Movement - Interactions – Performance, MIP, EA 4334, F-44000 Nantes, 3 cles and the occurrence of perineal tears as well as France. Poitiers University, INSERM, Poitiers university hospital, CIC 1402, Poitiers, France. 4INSERM, Center for Research in Epidemiology and the mode of delivery. Due to the expected number of Population Health (CESP), U1018, Gender, Sexuality and Health Team, women, it will not be possible to conclude about such University Paris-Sud, UMRS 1018, Orsay, France. 5Health and Rehabilitation an association. The objective is to provide preliminary Research Institute, Faculty of Health and Environmental Sciences, Auckland University of Technology, Auckland, New Zealand. data about the distribution of pelvic floor muscles elastic properties according the stage of perineal tear. Received: 12 September 2019 Accepted: 7 May 2020 We expect that these preliminary data would allow the future implementation of a larger multicentric References prospective study investigating the interest of includ- 1. Fritel X, Gachon B, Desseauve D, et al. Anal incontinence and obstetrical ing the elastic properties of the pelvic floor muscles anal sphincter injuries, epidemiology and prevention. Gynecol Obstet Fertil Senol. 2018;46:419–26. in our risk prediction of perineal trauma. This is re- 2. Gachon B, Nordez A, Pierre F, et al. Tissue biomechanical behavior should quired to offer each pregnant woman personalized in- be considered in the risk assessment of perineal trauma at childbirth. Arch formation and an individualized preventive strategy. Gynecol Obstet. 2019;300:1821–6. 3. Van Delft K, Sultan AH, Thakar R, et al. The relationship between postpartum Oneprospectmightbeaselectiveuseofepisiotomy levator ani muscle avulsion and signs and symptoms of pelvic floor in high-risk women considering that this intervention, dysfunction. BJOG. 2014;121:1164–71. Gachon et al. BMC Musculoskeletal Disorders (2020) 21:305 Page 12 of 12

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